This invention relates generally to ultra filtration and, in particular, to tubular membranes coupled in series to present a filtration module and to a method of preparing such a module.
Semi-permeable ultra filtration membrane devices have long been employed in both high pressure reverse osmosis and low pressure ultra filtration processes and systems. Typically, the membranes are formed into tubules and grouped together inside of a cylindrical vessel. Support members hold the tubules in spaced relationship and the assembly is referred to as a module. One or more modules may be used to concentrate or separate a wide variety of liquids and solids including polymeric emulsions or latices, body fluids, aqueous emulsions and low molecular weight solids such as urea and liquid solid systems such as a whey solution containing proteinaceous components. A typical material for constructing the semi-permeable tubular membranes is cellulose acetate.
The afore-mentioned modules may be arranged with their tubules in either series or parallel relationship depending upon the demands of the filtering process for which the module is used. Typically, a number of modules are joined together to accomplish the desired filtration.
There are known advantages to modules having their tubules arranged in series as opposed to a parallel arrangement. For example, where pressure drop is not a limiting factor, the pump sizing requirements for a given number of tubules arranged in series is substantially less than for the same number of tubules arranged in parallel. Another advantage of a series module is that cleaning of the inner surface of the filtration membrane may be accomplished by inserting properly sized spong balls into the fluid stream for passage through the tubules which make up the module. This sponge ball cleaning technique cannot be utilized reliably with a parallel arranged module since there is no assurance that fluid flow will carry sponge balls to each of the individual tubules.
There are, however, known disadvantages associated with a series arranged filtration module. The series arrangement requires a number of grommets and/or expanders as well as headers or U-bends, backup plates, and clamps. A typical series arranged ultra filtration module is shown and described in U.S. Pat. No. 4,309,287 issued Jan. 5, 1982. The large number of components which has heretofore been required for a series arranged filtration module not only increases the manufacturing cost of the module significantly over comparable parallel arranged modules, but also results in substantially increased potential for leakage with attendant increased maintenance costs. Another disadvantage of prior art constructions for series arranged ultra filtration modules is that the use of grommets, expanders and U-bends create uneven surfaces along the fluid path which can cause "hangup" of sponge balls utilized to clean the module. In applications where periodic sponge ball cleaning is required, however, the series arranged module is employed.